THE CONDUCTING CHANNELS 219 



the recognition of its first appearance easier. Strasburger mainly employed eosin, 

 but this pigment is retained by the tracheal walls, and, indeed, by all lignified 

 membranes \ 



As might be expected, experiments with dyes show that in certain tracheae 

 and tracheides water travels more rapidly than in others 2 , but in every case the 

 experiments with dyes only indicate that water travels rapidly in certain tissue- 

 elements. The possibility, however, always remains that in other elements which 

 the dye cannot penetrate water may also travel, though more slowly. Living 

 parenchyma, which is impermeable to a particular dye, might appear in such 

 researches to be quite inactive, and yet be really the seat of a rapid transference 

 of water. The results already given show, however, that the latter is not actually 

 the case. By other experiments, such as by injecting the vessels with gelatine, the 

 latter have been proved to be the active agents in the transport of water. 



The importance of the experiments with dyes is not lessened by the fact that 

 the water travels more rapidly than the dye. This phenomenon can be observed 

 when filter-paper is dipped into solutions of aniline dyes, and it is the necessary 

 consequence of the retention (cf. Sect. 28) of a certain amount of the dye, so that 

 the solution passes unchanged only when the saturation point of the wetted material 

 is reached. Hence experiments with dyes do not give the full rapidity of the water- 

 current. Nevertheless, when indigo-carmine or eosin is used the difference is not 

 very marked, and not much greater than when lithium is employed, the latter not 

 being perceptibly retained by the lignified walls. Owing to their much slower rates 

 of transference, methyl-blue, fuchsin, &c. are less adapted for such researches 3 . 



Rapidity of the water-current. The appearance of salts of Lithium, readily 

 recognizable by spectroscopic examination, was utilized by Sachs as a test for the 

 rapidity of water-transference in the conducting channels of a transpiring plant 4 . 



By watering the earth around potted plants with a i to 3 per cent, solution of 

 Lithium nitrate, a rapidity of transference of 0-18 to 2-1 metres per hour was observed. 

 Similar values were given by most of the plants used by Strasburger, although he 

 placed cut stalks in the solution and for the most part used a dye, eosin, as the 

 indicator 5 . In certain plants in which the water-currents are rapid, a transference 

 through as much as 6 metres was observed in an hour, with normal transpiration 

 (Bryonia, Cucurbita). These results were observed after the cut stems had remained 

 for a time in water, for if the negative pressure exhibited immediately after the stem 



1 The faulty methods and errors of Bokorny (Jahrb. f. wiss. Bot., 1890, Bd. xxi, p. 496) have 

 been sufficiently exposed by Hansen (Flora, 1890, p. 270) and Strasburger (Leitungsbahnen, 1891, 



P- 557). 



2 Wieler, Jahrb. f. wiss. Bot., 1888, Bd. xix, p. 566; Strasburger, 1891, 1. c., p. 566. 



3 Literature: Lehmann, Molecularphysik, 1888, Bd. I, p. 573; Goppelsrbder, Ober Capillar- 

 analyse, 1889; Sachs, Arb. d. Bot. Inst. in Wiirzburg, 1878, Bd. II, p. 157; Strasburger, Bau u. 

 Verricht. d. Leitungsbahnen, 1891, p. 550. 



4 Sachs, Arb. d. Bot. Inst, in Wiirzburg, 1878, Bd. II, p. 148. Lithium was first used by M c Nab, 

 Trans, of the Bot. Soc. of Edinburgh, 1871, Vol. xi, p. 45, and Trans, of the R. Irish Acad., 1874, 

 p. 343; later by Pfitzer (Jahrb. f. wiss. Bot, 1877, Bd. xi, p. 177). Pfitzer used salts of thallium, 

 M c Nab of caesium as well. On the poisonous properties of lithium, and its distribution in the 

 plant, see Gaunersdorffer, Versuchsst., 1887, Bd. xxxiv, p. 171. 



5 Strasburger, Bau u. Verricht. d. Leitungsbahnen, 1891, p. 588. 



